The invention relates to the field of hydraulic and pneumatic seals; more particularly it pertains to a dynamic seal for an assembly having a rotating component.
A type of a split rotating seal, widely known as a scarf-cut plastic seal, is used in many industries and is manufactured by a large number of companies worldwide. This type of seal is used in most modern automatic transmissions to seal shafts and clutch components against the passage of hydraulic fluid, usually transmission fluid.
In many cases, a scarf-cut plastic seal is made from PTFE compound, short slender fibers of glass, graphite or Kevlar, and a number of additives, the amalgam being a homogeneous mixture of PTFE compound, the fibers and additives. A purpose of the fibers is to reinforce the PTFE compound in order to improve its structural properties or to produce a seal whose functional characteristics are particularly suited to an application of the seal or to the environment in which the seal is intended to function. The additives have a similar purpose.
This type of seal typically has a rectangular cross section and a beveled split or scarf cut having an angle in the range of 7–20 degrees when projected on the longitudinal axis of the seal. This type of seal can be installed in an outer groove formed in the outer surface or journal of a rotating or stationary shaft. Alternatively, the seal can be installed in an internal groove formed in a bore of a rotating or stationary housing.
Installation of the seal in an internal groove is often preferable for packaging considerations. Also its installation in an internal groove is preferable for structural considerations because a groove formed in a shaft reduces the strength and stiffness of the shaft, and is an important factor when the shaft carries a structural load, especially a cyclic load. When the seal is installed in an internal groove and is pressurized by hydraulic fluid, the radial inner surface of the seal is expected to have a tight sealing surface contact with the outer surface of the shaft. When the seal is installed in an internal groove, a lateral face of the seal is expected to have tight seal surface contact with the adjacent lateral face of the groove. Also, clearance must be provided between the outer surface of the seal and the outer surface of the groove.
Internal groove, scarf cut, plastic seal installation is not commonly used in high volume applications due to the potential for damage to the seal during installation of a shaft. Scarf cut seals tend to loose their cylindrical form due to radial compression, which results in plastic deformation. They become dislocated in the retaining groove due to clearance between the surfaces of the seal and the groove, and they are susceptible to deformation. These factors cause unintended interference between the installed seal and shaft as the shaft is inserted within a bore. The seal is often cut by the shaft during shaft installation, commonly referred to as a “cut seal” condition. Or the seal is unintentionally forced from its groove by the shaft as the shaft slides in the bore past the installed seal. FIG. 2 shows an example of a conventional scarf cut seal from the prior art installed in an internal groove and contacted by a shaft while the shaft is moved during installation within a bore.
Avoiding the propensity for scarf cut seals, installed in a groove of an assembly, to sustain damage and to dislocate during installation of other components of the assembly is a long felt need. It is desired to overcome the difficulties of assembly so that scarf cut seals can be used reliably in high volume production and assembly.